"Tina" wrote in message
...
I understand how positive dihedral helps dynamic stability in
airplanes, but some big ones, like the Russian An 124 Condor, has a
pronounced negative dihedral -- the wings have a noticeable downward
slope.

Q1: Do those airplanes need active fly by wire controls to maintain
stability, or is something else at play that keeps them right side up?

Q2: Does anyone have a design rationalization for such a
configuration, as opposed to just zero dihedral? I can appreciate why
fighters have it -- they exploit lack of aerodynamic stability for
rapid maneuvers -- but transports that spend their whole life being
straight and level are another issue.

Note: I have not morphed into an Mx clone!

Dihedral (or anhedral - negative dihedral) are both used to reposition the
aircraft's Center of Lift (CL) vs the aircraft's CG. This effectively
creates a pendulum, which wants to stabilize with the heavy part at the
lowest possible position. Like a pendulum, relatively speaking, the farther
above the CG the CL is, the more stable an aircraft will be. The whole
"dihedral increases roll stability" issue is based around this, not
increased or decreased roll due to lifting forces stemming from the attitude
of the airplane. The roll forces which create stability are due to this
pendulum effect.

If you want a stable aircraft in roll, add as much dihedral as possible to
raise the CL. If you want an unstable aircraft, do the opposite. The
anhedral on some on high wing transports is probably designed to give a
desired amount of stability. Some level of stability is great for a
transport, but you don't want to create an aircraft so stable it requires
oversized ailerons to generate the desired roll rate.

KB